Half a croissant, on a plate, with a sign in front of it saying '50c'
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Cold Superheated Steam Food Dehydrator

Lower energy alternative to freeze drying food
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Almost every part of the world uses drying to preserve food. While quite effective, it's also quite energy intensive. According to Wikipedia, in some industrialized countries, as much as 12% of industrial energy is consumed in drying processes.

My idea uses a partial vacuum (like freeze drying), but with pressures and temperatures which aren't as low as those used in freeze drying.

The first part of the idea is to put the food being dehydrated on racks which allow gas circulation; there will probably be blowers or fans to improve convection even more. Actually, any equipment normally used for hot air drying would probably work.

This racks are then put into a pressure vessel, which is then sealed.

Gas (air or steam) from this vessel is circulated via a blower past a heat exchanger, where the gas acquires heat, then back into the vessel. This acquired heat is passed on to the food being dehydrated, which of course causes further drying.

Gas from the vessel is, simultaneously, moved via compressor into the inside of the heat exchanger; compression increases the temperature of the gas, and the heat exchanger cools the gas. After the initial air removal stage, the gas will be almost pure steam; the cooling of steam will of course condense it to water.

The fluid then flows from the heat exchanger into a gas/liquid separator. A float valve determines whether to remove liquid (via a condensate pump) or gas (via a vacuum pump).

The temperature is controlled as follows:

When the temperature in the vacuum chamber is higher than desired, the main compressor is cycled off. Gas will continue to flow into the heat exchanger, but via a check valve which bypasses the compressor. As the condensate pump and vacuum pump remove fluids from the separator, the overall system pressure and temperature drop, eventually causing the compressor to cycle back on.

When the temperature in the vacuum chamber is lower than desired, the vacuum pump is deactivated (or valves are used to allow it to assist the compressor at moving gas into the heat exchanger), and, if necessary, an auxiliary heater is activated to heat the gas that has been blown past the outside of heat exchanger and is about to enter the vacuum chamber. By not removing gas from the system, and by adding heat, the overall temperature and pressure rise.

For maximum energy efficiency, the thermostat should be set to allow a wide temperature range.

This idea has one big advantage over both regular hot air drying and freeze drying -- namely, it's much more energy efficient.

There's also one big advantage over regular hot air drying -- the temperature is much lower. Many foods are damaged by heat, and this idea avoids that.

The idea has one big disadvantage compared to hot air drying -- the equipment will cost more to construct, and it's hard to say how long the energy payback would be. The cost of equipment will be quite similar to that of a freeze-dryer.

The idea has one disadvantage compared to freeze drying -- in freeze drying, there's a phase change of the food's liquid to a solid, followed by desublimation, which a different effect from evaporation, due to the lack of surface tension during desublimation. This idea (like conventional hot air drying) uses evaporation, not desublimation, so some things will shrink as they dry which wouldn't occur with freeze drying.

goldbb, Jul 17 2011

The basis for this idea Cold_20Mechanical_2...Compression_20Dryer
[goldbb, Jul 17 2011]

[link]






       I thought dehydrating food sort of went out of style after WW2.. no?
bob, Jul 17 2011
  

       Beef jerky. Dried fruit. Pasta. Etc...
Alterother, Jul 17 2011
  

       rice, prunes, raisins, kidney-beans, flour, trail-mix...
FlyingToaster, Jul 18 2011
  
      
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